Generally, research on piezoelectric transformers is based on practical research conducted by C. E. Rosen, et al. in GE in the U.S. in 1957. Since the piezoelectric material used at that time was barium titanate, having a boosting range of 50 to 60 times under no-load conditions, there is the limitation of utilization of the piezoelectric transformer. However, a new piezoelectric material using Pb(Zr,Ti)O3 as a principal component is discovered, so that boosting over a wider range becomes possible, and thus research on the practical use of the piezoelectric transformer has recently been conducted in earnest.
When a conventional winding type transformer is compared to a piezoelectric transformer, the winding type transformer takes a boosting method based on electromagnetic induction, but the piezoelectric transformer uses piezoelectric and reverse piezoelectric effects, and thus causes less electromagnetic noise. Further, the boosting ratio is determined by a winding ratio in the case of the winding type transformer, but is determined by the characteristics of the material and the structure and dimensions of an electrode in the case of the piezoelectric transformer.
Further, from the standpoint of output power, since the winding ratio must be increased to transform the voltage and current of a secondary side into high voltage and low current in the case of the winding type transformer, a leakage component increases in proportion to the increase in the winding ratio. The piezoelectric transformer using a piezoelectric material exploits electrical-mechanical (primary side)-mechanical-electrical (secondary side) combination, and thus there is an advantage in that 90% or more efficiency can be realized.
Such a transformer, having high voltage and low current on the secondary side, can satisfactorily realize impedance matching when a load has high impedance, so that efficiency is improved, thus obtaining excellent load characteristics and increasing energy conversion efficiency.
The implementation of a piezoelectric transformer having high voltage and low current characteristics is still performed very satisfactorily, and the application of the piezoelectric transformer to the inverter for the backlight of a notebook computer, to which the piezoelectric transformer can be applied, has been positively considered.
Recent notebook computers have a trend toward small size and thinness, but a conventional winding type transformer is limited in usefulness when realizing small size and thinness due to the obtainment of insulation and resisting voltage, and decreases efficiency due to winding loss, such as core loss. In order to overcome such a disadvantage, if a piezoelectric transformer is applied to the LCD backlight of a notebook computer, impedance matching between the piezoelectric transformer and the LCD backlight of the notebook computer is satisfactorily performed, so that there is no problem in the application of the piezoelectric transformer to the backlight inverter of the notebook computer, and some products to which the piezoelectric transformer is applied have already been marketed.
Meanwhile, research on high output power, such as in a primary Rosen type and a tertiary Rosen type, has recently been actively conducted, and research on a layering method for parallel operation has been conducted therewith.
The problem of the basic structure of a conventional piezoelectric transformer is described with reference to FIG. 1. As shown in FIG. 1, various types of structures have been proposed as the basic structure of a piezoelectric transformer, but the structure of FIG. 1(a), having a rectangular planar shape, is evaluated as the most useful structure, from the standpoint of a manufacturing process and a boosting ratio.
As shown in FIG. 1, half of a planar piezoelectric element has an electrode formed in a thicknesswise direction and has a polarization direction also formed in the thicknesswise direction. The remaining half thereof has an electrode formed in a lengthwise direction and has a polarization direction also formed in the lengthwise direction.
In this case, the electrode part formed in the thicknesswise direction is designated as a driving unit, and the electrode part formed in the lengthwise direction is designated as an output unit. They correspond to the primary side and secondary side of the transformer, respectively.
When the input voltage of an inherent resonant frequency, determined to be 2L, which is a longitudinal dimension, is applied to the driving unit (thicknesswise electrode part), strong mechanical vibrations occur due to the electrostrictive effect, so that charges are generated because of the piezoelectric effect, occurring in the output unit (lengthwise electrode part) due to such mechanical vibrations, and high AC voltage can be obtained at an output terminal.
That is, a boosting operation is performed using transformation from electrical energy to mechanical energy or from mechanical energy to electrical energy.
FIG. 2 illustrates the distribution of displacement and stress in respective vibration modes.
In FIG. 2, the point at which displacement is 0 and stress is maximized is called a node point, and the maximal boosting ratio can be obtained only when the node point is supported.
The greatest disadvantage of this structure is in that, since the polarization directions of the driving unit and the output unit are orthogonal, the concentration of stress on a boundary surface becomes serious, in that, since a high electric field of 3 kV/mm is applied when the output unit is polarized, the implementation of polarization is difficult, and in that, since the electrode area of the output terminal is small, it is difficult to obtain high current, and thus high voltage and low current output characteristics are obtained. Accordingly, such a structure is not suitable for high current lighting, such as in a fluorescent lamp.
In order to solve this problem, a method of independently manufacturing a plurality of piezoelectric elements and driving the piezoelectric elements in parallel has been proposed. However, this method is problematic in that, since it is difficult to control the precise dimension of electrodes, resonant frequencies cannot be identical to each other, and since the disadvantage of the polarization process is not overcome, output characteristics are deteriorated.